<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    html {
      height: 100%;
    }

    body {

      display: flex;
      flex-flow: column;
      height: 100%;
      background-color: yellowgreen;
    }

    .row {
      width: 100%;
      height: 50px;
      background-color: red;
      line-height: 50px;
      text-align: center;
      font-size: 20px;
    }

    section {
      flex: 1;
      overflow: auto;
    }
  </style>

</head>

<body>
  <div class="row">客服系统</div>
  <section>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Accusamus omnis, maxime aliquid sapiente eius unde
    soluta laboriosam molestias nemo enim alias vitae assumenda dicta nihil dignissimos nisi, aspernatur ratione
    doloremque?</section>
  <div class="row">客服沟通中</div>
</body>

</html>